1. Field of the Invention
The present invention relates to a method for producing a hollow molded article, preferably a multilayer hollow molded article, which is widely used as a back and a seat of a chair, a cold box and the like.
2. Description of the Related Art
A widely used method for producing a hollow resin article is a blow molding method which includes pressurizing air in a parison in a molten state contained in a female mold to expand the parison in conformity with a shape of an inner wall of a mold cavity. Though this method is advantageous in the absence of a male mold, it is impossible to form a non-uniform wall thickness of the parison. Then, this method is suitable for the production of an article having a symmetric cross section such as a bottle, but not for a profile article having an incomplete symmetric cross section. In addition, the molded article has a flaw, namely a parting line at a part corresponding to a parting plane of the female mold and sometimes burrs are formed at the flaw.
To produce a profile article such as a back of a chair, hollow molding has been developed by modifying injection molding.
For example, in one method, a high pressure gas is pressurized in a molten state resin through a hole bored in a wall of a mold to form a gas passage, namely a hollow part in the resin. However, this method has various disadvantages that a large pressurizing mark having a size of about 3 mm is formed in the produced article, that color irregularity appears between a surface of a gas penetrated part and a surface of a gas non-penetrated part, that since the gas passage is deviated, the hollow part is also deviated in the article, that is, a wall thickness of the article becomes uneven, that a diameter of the hollow part is not uniform, that a resin bank is formed in the article, and that if the injection pressure is too high, bubbles are formed in the wall of the article.
There is a method in which a high pressure gas is pressurized in a molten resin which is supplied from a cylinder nozzle through a gas-injection inlet provided in the nozzle. Since only a position for gas injection is different from the above method, this method has substantially the same disadvantages.